1. Field of the Invention
The present invention relates to a container exhibiting an excellent sealing performance and capable of being easily opened and sealed and a method of and apparatus for notching the same.
2. Description of the Related Art
Hitherto, containers of the following type for packing food are widely used which comprises a main container body having a flange portion and a cover, this flange portion being welded to the cover for sealing. In order to make such containers to attain both a capability of being readily opened and an excellent sealing performance, they are designed in such a manner that the adhesive force acting between layers of the container which is formed to be a multilayered structure is arranged to be smaller than the adhesive force acting between the flange portion and the cover. Furthermore, a non-sealing portion is formed inside this sealing portion and a notch which serves as a weakening line is formed in this non-sealing portion so that the layers forming the main container body are separated from each other from the outer periphery thereof to the notch when this container is intended to be opened. Furthermore, in order to improve the rigidity of such containers, there are containers of the type having a flange portion in which a bent portion such as a rib, a skirt, a curl or the like is formed. In the container of the type described above, a cutting line is formed as to serve as an weakening line on the outer periphery of the sealing portion in order to improve the capability of being readily opened so that the layers forming the main container body are separated from each other starting from the thus-formed cutting line.
However, in the case where the cutting line is previously formed in the flange portion, it is difficult to accurately perform the welding for sealing at the position of this cutting line. If the outer periphery of the sealing does not meet accurately in the inner or outer portion of the cutting line, the opening of the container becomes difficult to be performed.
To this end, a container capable of overcoming the above-described problem is proposed (Japanese Patent Laid-Open No. 63-96060), this container being arranged such that the rib in the opening portion is cut out as to meet the cut surface in the outer periphery of the flange portion and the outer periphery of the sealing portion.
However, in this conventional example, the following problems arisen:
(1) since the rib in the flange portion is cut off, the rigidity and the strength in this cut portion can deteriorate, in addition
(2) edges can be adversely broken on both sides of the cut portion when the container is opened, and the peeling remainder is generated in the opened surface, causing the appearance of the container to deteriorate, and
(3) since the outer periphery of the sealing is made to meet the cut surface of the outside end of the flange portion, the sealing work cannot necessarily be conducted easily and the width of the sealing portion cannot be optionally determined.
If a material having a proper sealing strength were selected for the purpose of improving the capability of being readily opened, the sealing performance can inevitably changed due to the sealing conditions such as temperature, pressure, time and the like, and impurities such as a filler. Therefore, a uniform adhesive force cannot be obtained, and the sealing performance deteriorates. As a result, such a container cannot be used for packing retort foods whose inner pressure is involved to be raised for disinfection by heating.
Therefore, the applicant of the present invention has disclosed a structure (Japanese patent Publication No. 61-229591) arranged such that the container is designed to be a multilayered structure comprising a cover, an inner layer (a first layer) to be sealed to the cover, and an outer layer (a second layer) to overlap the inner layer, and the inner layer can be separated from the outer layer. Furthermore, a notch is formed in the inner layer of a non-sealing portion in the flange portion so that the container is opened by separating the inner layer from the outer layer starting from this notch.
The container of the type described above needs to be provided with the notch in the inner layer of the flange portion thereof The following method and apparatus can be employed for forming a notch of the type described above:
Art I (Japanese Patent Publication No. 61-229591) in which the notch is formed, in the manufacturing process for the container, by pressing an annular notching blade to the flange of the container with this flange received by a receiving member such as a mold; and
Art II (Japanese Patent Publication no. 61-229592) in which the notch is formed by means of pressing by using the notching blade and/or heating means.
However, in the above-described Arts I and II, since the flange of the container needs to be received by a mold or the like for forming the notch, the notch can be formed only in the flat portion of the flange. It is difficult to apply these arts to a flange having a bent portion such as a rib or a curling. In addition, the formed notches become non-uniform if the thickness of the flange is not uniform. Furthermore, in the Art I the cost for manufacturing the annular notching blade is expensive, the notch cannot be formed in the flange portion cheaply.